The invention relates to a method for providing a supply voltage and to an electrical drive system, in particular when supplying voltage to electric machines with a hybrid energy source system from a fuel cell and an electrical energy store.
It is apparent that, in the future, electronic systems which combine new energy storage technologies with electrical drive technology will be used more and more both in fixed applications such as, for example, wind power plants or solar plants, as well as in vehicles such as hybrid vehicles or electric vehicles.
In order to feed three-phase current into an electric machine, a direct voltage which is provided by a direct voltage intermediate circuit is conventionally converted into a three-phase alternating voltage by means of a power converter in the form of a pulse-controlled inverter. The direct voltage intermediate circuit is fed from a line of battery modules connected in series. In order to be able to satisfy the requirements placed on a respective application in terms of power and energy, frequently a plurality of battery modules are connected in series in one traction battery. Such an energy storage system is frequently used, for example, in electrically operated vehicles.
Document U.S. Pat. No. 5,642,275 A1 describes a battery system with an integrated inverter function. Systems of this type are known by the name Multilevel Cascaded Inverter or else Battery Direct Inverter (BDI). Such systems comprise direct current sources in a plurality of energy storage module lines which can be connected directly to an electric machine or an electrical power system. In this context, single-phase or multi-phase supply voltages can be generated. The energy storage module lines have here a multiplicity of energy storage modules which are connected in series, wherein each energy storage module has at least one battery cell and an assigned controllable coupling unit which permits the respective energy storage module line to be interrupted as a function of control signals, or the respectively assigned at least one battery cell to be bypassed or the respectively assigned at least one battery cell to be connected into the respective energy storage module line. By means of suitable actuation of the coupling units, for example using pulse width modulation, suitable phase signals for controlling the phase output voltage can also be provided, with the result that it is possible to dispense with a separate pulse-controlled inverter. The pulse-controlled inverter which is necessary to control the phase output voltage is therefore integrated into the battery.
As an alternative, documents DE 10 2010 027 857 A1 and DE 10 2010 027 861 A1 disclose battery cells which are connected in a modular fashion in energy storage devices which can be selectively coupled into the line of battery cells connected in series, or decoupled therefrom, by means of suitable actuation of coupling units. Systems of this type are known by the name battery direct converter (BDC). Such systems comprise direct current sources in an energy storage module line, which direct current sources can be connected to a direct voltage intermediate circuit for supplying electrical energy to an electric machine or an electric power system via a pulse-controlled inverter.
An alternative possible way of generating electrical energy for an electrical drive system is provided by fuel cells. In this context, fuel cell vehicles are frequently equipped with additional electrical energy stores in order, for example, to provide a temporary increase in power for the drive or to permit recuperation of braking energy. Hybrid fuel cell systems of this type can have, on the one hand, direct voltage transformers which serve in order to couple the fuel cells and the electric energy stores and in order to approximate the voltage ranges and power fluxes of the hybrid energy sources. On the other hand, corresponding operating strategies can be selected for the fuel cells in order to perform adaptation of the voltage and power flux such as is shown, for example in documents EP 1 233 468 A2 and U.S. Pat. No. 7,829,229 B1.
There is therefore a need for cost-effective options, which can be effected with little expenditure in terms of technical implementation, for providing hybrid energy source systems composed of fuel cells and electrical energy stores for supplying voltage to electrical drive systems.